Recently, materials known as “metamaterials” have been discovered. Metamaterials include a type of artificial structure, referred to as a “left-handed medium” (LHM), which is characterized by having a negative refractive index. Generally, metamaterials are periodic structures composed of artificially constructed structural units, or “cells,” whose dimensions are smaller than the radiation that is being controlled, but are far larger than atomic or molecular scales. The metamaterial interacts with radiation as if it is a homogenous material with an effective electric permittivity and magnetic permeability. The goal is to design the sub-wavelength structures (i.e., the cells) to achieve properties and effects that do not arise from known natural materials, whose unit cells are atoms or molecules.
Simply put, a LHM affects electromagnetic waves by having structural features smaller than the wavelength of the electromagnetic radiation it interacts with. For example, if microwave radiation (with wavelength λ approximately 1 m to 1 mm) is used, the LHM needs to have a structure smaller than 1 mm. Microwave frequency metamaterials are constructed as arrays of electrically conductive and non-magnetic metal elements (such as loops of copper wire) which have suitable inductive and capacitive characteristics. In particular, these structures are designed to have strong coupling to magnetic fields at microwave frequencies with low losses. Such is not a characteristic of ordinary materials. For example, ordinary non-magnetic materials have extremely weak coupling to magnetic fields, and ferromagnetic materials have strong coupling but large losses. The most common type of metamaterials for microwave radiation are based on split-ring resonators.
FIG. 1 depicts a conventional split-ring resonator 1 according to the related art. The conventional split-ring resonator 1 has been shown to be effective at achieving a negative refractive index for tower-frequency radiation, such as microwaves. As shown in FIG. 1, the conventional split-ring resonator 1 is typically formed as a pair of concentric annular rings 2 and 3 with splits in them at opposite ends. The pair of concentric annular rings includes an inner ring 2, and an outer ring 3 wrapped around the inner ring 2, with the rings having gaps between them and having splits formed approximately 180 degrees apart from each other. Conventionally, the rings are made of an electrically conductive and nonmagnetic metal, such as copper.
The split-ring resonator is typically disposed in a substrate 4. The substrate 4 is usually a circuit board, or may be formed out of fiberglass or some other material. The substrate 4 typically supports a periodic, or repeating, array of conventional split-ring resonators 1 formed in parallel with each other and connected to each other by conductive wire strips 5.
A magnetic flux penetrating the metal rings 2 and 3 will induce current in the rings 2 and 3, which in turn produces a magnetic flux that can either enhance or oppose the incident field, depending on the resonant properties of the split-ring resonator and the frequency of the radiation 1. Due to splits in the rings 2 and 3, the structure can support resonant wavelengths much larger than the diameter of the rings. The small gaps between the rings produce large capacitance values which lower the resonance frequency. At frequencies below the resonant frequency, the real part of the magnetic permeability of the split-ring resonator becomes large and is positive, and at frequencies higher than resonance it becomes negative. The negative permeability response can be used with the negative dielectric constant of another structure to produce a “left-handed material” with negative refractive index. Left-handed materials can have very interesting and potentially very useful properties not found in naturally occurring materials.
The type of radiation which the conventional split-ring resonator 1 will effectively work with is limited by the dimensions of the split-ring resonator 1. The diameter of the conventional split-ring structure 1 must be very small compared to the resonant wavelength in order to achieve a negative refractive index. Microwaves typically have wavelengths ranging from 1 in to 1 mm, and corresponding frequencies ranging from 300 MHz (0.3 GHz) to 300 GHz. Accordingly, to effectively work with microwaves, the conventional split-ring resonator 1 has been designed to have a diameter of less than 1 mm. Continuing research is underway to design split-ring resonators with increasingly smaller diameters that will function with increasingly higher frequency and lower wavelength radiation, and split-ring resonators with diameters as small as a few dozen μm have been achieved.
However, a problem with all of the conventional split-ring resonators in the related art is that these conventional split-ring resonators employ copper or some other metallic wire to form the split rings. Metal wire is increasingly difficult to manipulate as the diameter or size decreases, and this presents a serious technical obstacle for creating split-ring resonators which will achieve a negative refractive index for higher frequency radiation, such as visible light, which has a wavelength range of about 380 nm-780 nm and a corresponding frequency ranging from about 430 trillion Hz (430 THz) to about 750 trillion Hz (750 THz). For a split-ring resonator to achieve a negative refractive index for optical frequencies, including visible light, a conventional split-ring resonator must have an inner radii of no greater than 30 to 40 nm, and preferably much less. In the related art, it has so far been impossible to design a split-ring resonator having the required inner radii to function at optical frequencies using conventional structures, such as copper split-rings. As a result, it has also been impossible to achieve the numerous potential benefits that would result from being able to manipulate higher frequency radiation with a negative index of refraction material, such as sub-wavelength resolution optics, optical cloaking, ultra-high efficiency detection, and likely applications for more powerful communications and computing.
Additional problems occur using metallic split-ring resonators. For example, naturally paramagnetic and ferromagnetic materials only achieve strong coupling to magnetic fields that have significant losses. Moreover, resonance for electric and magnetic response does not occur over the same frequency and for natural materials, whose response to electromagnetic fields is governed by individual atomic or molecular properties, there is no obvious way to change this.
The desirability of interacting with or operating at optical frequencies is well-established. For example, stronger magnetic coupling occurs which can achieve negative index of refraction in the optical range leading to super-lenses (sub-wavelength imaging) and potentially assisting the development of transformation optics through the complete control over light. Structures with strong magnetic coupling and low losses at optical frequencies, even without negative index of refraction behavior, would also be important in this regard.
Some scaling down of structures to produce metamaterials for optical frequencies using nano-scale fabrication has occurred. One approach has been to create very small metallic rods which are physically connected and fabricated using electron beam and nanolithography techniques.
Moving from a metallic split-ring resonator to non-metallic, photonic split-ring resonators is analogous to the progression of the technology associated with communications evolving from land-line based communications transmitted through copper wire, to radio signals transmitted through the air, to fiber optic light pulses transmitted through fiber optic cables, to digital wireless communications.